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LinkedIn now No. 2 social networking site

LinkedIn, the professional networking site, edged past MySpace last month to become the No. 2 social networking site in the United States, according to comScore.

MySpace, now the No. 3 site, has been struggling for awhile as more users gravitated to Facebook. MySpace, in the process of being sold by News Corp to Specific Media, is bringing on singer/actor Justin Timberlake to be the site's creative director to help invigorate it.

LinkedIn had 33.9 million unique visitors in June in the U.S., compared to MySpace's 33.4 million; Facebook had 160.8 million. Short messaging blog Twitter is closing in on MySpace, with 30.6 million, and Tumblr had 11.7 million.

"I think we're seeing the U.S. social networking market come of age," Andrew Lipsman, comScore's vice president of industry analysis said in an email to msnbc.com. "While Facebook has clearly emerged as the dominant player in this market, several other social networks — Linkedin, Twitter and Tumblr — are carving out strong niches.

"Importantly, many of these secondary players have reached critical mass with their audiences, and the networks effects have become strong enough to attract more people and higher engagement. This effect in part explains why we're seeing these sites post all-time highs nearly every month right now."

In a blog posting last month, before the new figures were released, Lipsman had noted how "the picture is not so rosy for MySpace, which continues to see attrition in its U.S. user base ... its audience has declined by nearly 50 percent in the past year while average user engagement has dropped 85 percent."

Also of interest was his comment that given Facebook's size — it now has 750 million users worldwide — that its future growth in the U.S. is "likely to come more from increasing usage per visitor than its ability to attract new users in perpetuity. One impressive stat to note is that Facebook’s average U.S. visitor engagement has grown from 4.6 hours to 6.3 hours per month over the past year, so it appears to be succeeding in that regard."

Whether the same will hold true for LinkedIn — where people tend to get in and out quickly to check or make connections — remains to be seen.